Conventionally, plasma processors that generate plasma to apply predetermined processing to an object to be processed by the action of the plasma have been often used.
For example, in a semiconductor device manufacturing field, etching and film deposition are conducted by the action of plasma on a substrate to be processed such as a semiconductor wafer or the like when a microscopic circuit structure of a semiconductor device is to be formed.
In such a plasma processor, since plasma is generated in a vacuum processing chamber, the temperature may possibly rise due to the action of the plasma. Therefore, many plasma processors are provided with a temperature control mechanism for controlling the temperature of a predetermined portion.
For example, in a plasma processor of a so-called parallel plate type that applies high frequency power between an upper electrode and a lower electrode facing each other to generate plasma, a mounting table (susceptor) for placing a substrate to be processed such as a semiconductor wafer thereon also functions as the lower electrode. A flow path of a heat medium is formed in a block made of a conductive material (for example, aluminum or the like) which forms the lower electrode, and an insulating fluid as the heat medium, for example, a fluorinated insulating fluid is circulated in the flow path for temperature control of the semiconductor wafer or the like.
Examples of the fluorinated insulating fluid are, Fluorinert (trademark: manufactured by Sumitomo 3M) (fluorinated inert liquid composed of carbon and fluorine), GALDEN HT (trademark: manufactured by Augimont) (perfluoropolyether composed of fluorine, carbon, and oxygen), and so on. These insulating fluids have a thermal conductivity (25° C.) of about 0.06 W/mk, a volume resistivity (25° C.) of about 1E17 to 1E18 Ω-m, and a permittivity (25° C., 1 kHz) of 1.5 to 2.0.
Further, in many of plasma processors as structured above, an electrostatic chuck is provided on the block made of aluminum or the like which constitutes the aforesaid lower electrode, and the electrostatic chuck holds a semiconductor wafer or the like by suction. Such an electrostatic chuck is formed such that an electrode for electrostatic chuck is disposed in an insulating film made of an insulative material. Further, in order to achieve high efficiency and accuracy of the aforesaid temperature control of a semiconductor wafer or the like, many plasma processors have a cooling gas supply hole passing through the lower electrode and the electrostatic chuck and cooling gas such as helium is supplied to a rear surface side of the semiconductor wafer or the like from the cooling gas supply hole.
In the above-described plasma processor, if there is time before processing of a subsequent lot after the completion of processing of some lot, the processor is set on standby (idle state) to prevent particles from staying in the vacuum processing chamber and moisture from adhering to a wall surface and the like of the vacuum processing chamber so that processing can be started in a clean environment when the lot to be processed next is transferred thereto.
In such an idle state, conventionally, the vacuum processing chamber is supplied with inert gas such as nitrogen gas at a predetermined flow rate and is vacuumized while undergoing no pressure control. As a result, the pressure in the vacuum processing chamber is, for example, 5 Pa or lower.
Meanwhile, the circulation of the insulating fluid in the flow path of the lower electrode is not stopped but is continued
However, if the idle state as described above continues for a long time, the circulation of the insulating fluid causes the generation of electrostatic due to friction between an inner wall of the flow path and the insulating fluid and electric charges are accumulated in the lower electrode. Further, since an upper portion and so on of the lower electrode are covered with the insulative material for electrostatic chuck, the accumulation of the electric charges results in increase in charged voltage. FIG. 6 shows the result of measuring such a time-dependent increase in charged voltage, the solid line F in FIG. 6 showing the result when the flow rate of the insulating fluid is set to 201/minute and the solid line G showing the result when the flow rate of the insulating fluid is set to 301/minute.
As shown in the drawing, in both cases where the flow rate of the insulating fluid was set to 201/minute and 301/minute, the charged voltage increased with time to reach 8000 V or higher. Further, as the flow rate of the insulating fluid was higher, the charged voltage increased in a shorter time, i.e., it took about 3 hours for the charged voltage to reach 8000 V or higher in the case of the 301/minute flow rate while about 8 hours in the case of the 201/minute flow rate.
For reference, the result of similar measurement of the charged voltage under varied temperatures of 0° C., 20° C., and 40° C. of the insulating fluid showed that the charged voltage becomes higher as the temperature of the insulating fluid becomes higher. This is supposed to be because lowered viscosity due to the temperature rise of the insulating fluid increases the velocity of the insulating fluid, so that an amount of electrostatic generated by the friction between the inner wall of the flow path and the insulating fluid becomes larger.
Further, it was found from a breakdown test that electric discharge occurred between the electrode of the electrostatic chuck and the lower electrode (aluminum block) at an instant when the charged voltage reached about 8000 V, which may possibly cause breakdown of the insulating film of the electrostatic chuck, as shown in FIG. 7.
Incidentally, the increase in the charged voltage as described above can be prevented by switching the potential of the lower electrode to a ground potential during the idle state, but if such a switching operation is not performed surely, there is a possibility of breakdown or the like of the insulating film of the electrostatic chuck due to the increase in the charged voltage as described above.